title: Catalytic self-acylation of type II polyketide synthase acyl carrier proteins.
abstract: BACKGROUND
  Aromatic polyketides are synthesised in streptomycetes by the successive condensation of simple carboxylic acids, catalysed by multienzyme complexes--the polyketide synthases (PKSs). Polyketide assembly intermediates are covalently linked as thioesters to the holo-acyl carrier protein (ACP) subunit of these type II PKSs. The ACP is primed for chain elongation by the transfer of malonate from malonyl CoA. Malonylation of fatty acid synthase (FAS) ACPs is catalysed by specific malonyl transferase (MT) enzymes. The type II PKS gene clusters apparently lack genes encoding such MT proteins, however. It has been proposed that the MT subunit of the FAS in streptomycetes catalyses malonylation of both FAS and PKS ACPs in vivo.
  
  
  RESULTS
  We demonstrate that type II PKS ACPs catalyse self-malonylation upon incubation with malonyl CoA in vitro. The self-malonylation reaction of the actinorhodin C17S holo-ACP has a K(m) for malonyl CoA of 219 microM and a kcat of 0.34 min-1. Complete acylation of the PKS ACPs was observed with malonyl, methylmalonyl and acetoacetyl CoAs. No reaction was observed with acetyl and butyryl CoAs and FAS ACPs did not react with any of the substrates. Recombinant FAS MT from Streptomyces coelicolor did not accelerate the rate of malonylation.
  
  
  CONCLUSIONS
  The catalytic self-acylation of type II PKS ACPs is an unprecedented reaction. We propose a reaction mechanism in which conserved arginines form a salt bridge with the acyl moiety and sequester it from bulk solvent. This work suggests that the beta-ketoacyl synthase, chain length factor and ACP may constitute a truly minimal PKS in vivo.
authors.affiliation: School of Chemistry, University of Bristol, UK.
authors.forename: T S
authors.initials: TS
authors.lastname: Hitchman
authors.forename: J
authors.initials: J
authors.lastname: Crosby
authors.forename: K J
authors.initials: KJ
authors.lastname: Byrom
authors.forename: R J
authors.initials: RJ
authors.lastname: Cox
authors.forename: T J
authors.initials: TJ
authors.lastname: Simpson
mesh_terms: D000213:Acyl Carrier Protein / Q000737:chemistry*; D000217:Acyltransferases / Q000378:metabolism; D064429:Fatty Acid Synthases / Q000378:metabolism; D008316:Malonyl Coenzyme A / Q000378:metabolism; D013058:Mass Spectrometry; D008958:Models, Molecular; D009097:Multienzyme Complexes / Q000737:chemistry / Q000378:metabolism*; D013302:Streptomyces / Q000201:enzymology* / Q000378:metabolism
chemical_list: D000213:Acyl Carrier Protein; D009097:Multienzyme Complexes; D008316:Malonyl Coenzyme A; D000217:Acyltransferases; D064429:Fatty Acid Synthases